The present invention relates generally to a microwave oven of the type including a duty cycle control which periodically energizes and deenergizes the microwave generating system during the cooking process, and more particularly to an improved start circuit arrangement providing a delay start feature in such an oven.
It is common in commercially available electronically controlled microwave ovens to provide a delay start feature. Such a feature permits the microwave oven user to program the control circuitry of the oven so that a cooking operation begins at a later time. The desired delayed starting time is selected by user actuation of the appropriate touch panel keys. Typically, the user of the oven, when using the time delay feature, places the food in the oven cavity, first selects the time delay mode and enters the desired starting time for the cooking cycle, then enters the desired cooking mode or modes and the time durations for each selected mode. Following completion of entry of the time delay and cooking cycle instructions, the user then presses a start button and leaves the oven unattended.
Duty cycle control is generally utilized to control the output power of the microwave oven magnetron, resulting in the magnetron being switched between a full ON condition and a full OFF condition with the percentage of ON time being varied to change the cooking power level for the food being heated. Hence, the magnetron is a load in the oven which is energized intermittently during the cooking process. In addition, the microwave oven also includes other electrical loads which are energized continuously during the cooking process, including a magnetron cooling blower, an oven lamp and perhaps a mode stirrer motor.
Provision is usually made to control the continuously energized loads separately from the magnetron since the same switching device, which is an intermittent or duty cycle controlled device, cannot be used to control the continuously energized loads.
This presents a particular problem in providing a delay start feature. One solution to this problem is to turn on the constantly energized loads when the oven user completes entry of the delay start instructions. However, in such an arrangement these loads such as the mode stirrer motor, lamp and the blower would be energized by actuating the start button or similar input device and remain continuously energized during the delay time period when the magnetron is idle, waiting for the beginning of the cooking cycle. The obvious disadvantage of such a scheme is that power used to drive these loads is wasted during the time when they are not functionally needed to support the cooking operation. In addition noise caused by operation of these loads is undesirable and distractive.
Commonly assigned U.S. Pat. No. 4,345,135 to Larry Harmon discloses an improved delay start arrangement for a microwave oven, which utilizes a delay start relay in addition to the power control relay. The delay start relay coil is arranged to respond to the initial turn on of the magnetron by the magnetron switching element. This delay start relay, when energized, closes a holding current path for its coil to maintain it in an energized condition during the remainder of the cooking cycle. Energization of this time delay relay enables energization of the various continuously energized loads in the circuit. In the Harmon arrangement the power relay is closed by actuation of the start switch and energization of the magnetron is enabled at the beginning of the cooking cycle by trigger signals applied to the magnetron switching element from the electronic control. The time delay relay responds to the initial flow of current to the magnetron to couple the continuously energized load portion of the circuit. At the end of the cooking cycle the electronic control generates a signal to open the power relay which terminates the cooking operation and deenergizes the delay start relay.
The present invention is an improvement over the Harmon arrangement, providing a delay start capability using electronic circuitry thereby eliminating the bulky and relatively expensive delay start relay. The electronic circuitry is arranged to prevent the erroneous energization of the oven magnetron as a result of a single point failure in the electronics.